如何将 IBM 浮点数转换为 JavaScript 数字？

Question

I want to read data from in an old file format. The format uses IBM floating point numbers, and I want to read it in a JavaScript program. IBM floating point representation was used on the IBM System/360 .

How can I convert the IBM floating points to JavaScript numbers?

Answer 1

This function takes a buffer of little endian IBM floats and converts them to a number:

function ibm2js(buffer) {
    var sign = buffer[0] >> 7
    var exponent = buffer[0] & 0x7f
    var fraction = 0
    function bit(buffer, bit) {
        return buffer[Math.floor(bit / 8)] >> (7 - (bit % 8)) & 1
    }
    for (var i = 0; i < 24; i++) {
        fraction += bit(buffer, 8 + i) / (2 << i)
    }
    return (1 - 2 * sign) * Math.pow(16.0, exponent - 64) * fraction
}

Note that special cases like NaN, Infinity, etc. is not covered.

This was ported from: http://objectmix.com/perl/21834-ibm-32-bit-floating-point-conversion.html

Answer 2

A small update to Hallgrim's solution, which allows to handle double-precision floats:

function ibm2js(buffer) {
    var sign = buffer[0] >> 7
    var exponent = buffer[0] & 0x7f
    var fraction = 0
    function bit(buffer, bit) {
        return buffer[Math.floor(bit / 8)] >> (7 - (bit % 8)) & 1
    }
    var denom = 1
    var totalBit = (buffer.length - 1) * 8
    for (var i = 0; i < totalBit; i++) {
        denom = denom * 2
        fraction += bit(buffer, 8 + i) / denom
    }
    return (1 - 2 * sign) * Math.pow(16.0, exponent - 64) * fraction
}